When my ds was a baby, we were on a road trip that for the trip back home was done at night. Since it was at night, we had figured that ds would be asleep, especially since we were in the car where he'd be more likely to fall asleep. Unfortunately, that is not how things happened. He stayed awake. Very awake. Very unhappy and awake. Seems that the lights from the headlights of the other cars on the highway were falling right into his eyes. I would have been awake and unhappy, too, in that situation! I do hope that you don't have that situation, and if you do, you are able to pull over and find somewhere for your whole family to sleep.
I second the idea of pipecleaners and a aquadoodle. A magnadoodle is good, too. For your three year old, bring along some tape. It amazes me how long that can entertain a kid of that age, especially when combined with pipecleaners (and straws!). If you are comfortable with him using scissors in the car (think those plastic safety scissors used in preschools), bring along some old magazines and he can make collages. I would rethink the gameboy unless your ds already has experience with them only because you may be setting him up for great frustration when he can't do something fast enough in order to get anywhere, or can't figure out how to play the game at all. Unless of course you don't mind turning around to the back seat every 30 seconds to help him! (BTDT, and almost tossed the game out the window).
I would plan on the potty breaks being longer than a quick in and out. Your 1yo in particular is going to need it, as there is nothing they like at that stage more than being able to move about! You will also need to make sure that both you and your dp have the time to sleep so that you can be well-rested when it your time to drive. Driving while sleepy is seriously not safe for anybody!